Last big chill suggests lower climate impact of carbon

Researchers use the conditions that prevailed during the peak of the last …

One of the key measures of the impact of atmospheric carbon dioxide is called the climate sensitivity, which provides an estimate of how much the planet will warm in response to a doubling of the CO2 concentration. This figure has been estimated using a variety of methods, producing a range of values; the Intergovernmental Panel on Climate Change estimates that the most likely value is 3 Kelvin, but recognizes there's a reasonable chance it could range anywhere from 2.4-4.5K. A new study that uses a climate model to evaluate the peak of the last glacial period, however, suggests that the IPCC's figure might be a bit high, and that very high values are overwhelmingly unlikely.

Glacial periods are triggered by small changes in the Earth's orbit. These aren't enough by themselves to alter the global climate, but they set off a drop in atmospheric CO2 and an expansion of ice, which reflects sunlight back to space. These feedbacks help the Earth enter a deep chill during glacial periods.

The study focuses on the peak of the last glacial cycle, called the Last Glacial Maximum, which took place around 20,000 years ago. The conditions the authors use include larger ice sheets, lower greenhouse gas concentrations, increased dust, and the changes in solar forcings driven by the orbital differences (a forcing is anything that can shift the climate). For the actual conditions, they obtained temperature data from pollen samples, ice cores, and ocean sediments. Combined, these samples cover about a quarter of the planet's surface, and provide one of the most detailed reconstructions of the temperature of the LGM.

So the authors have a set of inputs—the things that force the climate—and an estimate of the output, namely the global temperature. To estimate the climate sensitivity, they perform multiple runs of a suite of climate models, providing them different climate sensitivities, and perform a Bayesian analysis to figure out which values are consistent with the state of the planet. For this work, they used a set of 47 different versions of the University of Victoria climate model, which they consider "intermediate complexity." In this case, that means that it incorporates information on dust, but doesn't include feedback changes in clouds or winds.

To look at climate sensitivity, the authors short-circuited the actual role of carbon dioxide, and simply changed its impact by adjusting the amount of infrared radiation that escapes through the atmosphere (carbon dioxide acts by trapping this radiation). Each of the 47 different models has a different value for this escaping radiation, and so models different levels of greenhouse gas impact.

This approach let them set a number of limits on the climate sensitivity. For example, model runs where it was too low keep the planet warmer than it was at the LGM. In other words, if the contribution of reduced CO2 levels is too small, the changes in the remaining forcings aren't enough to trigger a deep glaciation. In the same way, high climate sensitivities produce an extremely cold planet, far colder than the LGM. In fact, climate sensitivities above 6K trigger a global glaciation, or snowball Earth—something that has happened in the past, but not for over half a billion years. "Our model thus suggests that large climate sensitivities cannot be reconciled with paleoclimatic and geologic evidence, and hence should be assigned near-zero probability," they conclude.

Overall, their best fitting model involved a climate sensitivity of 2.4K, a touch under the IPCC's best fit, and a range of likely values that's also generally lower than the IPCC's. That best fit, however, has some problems: the model has Antarctica 4K warmer than it actually was, but suggests the West Antarctic Ice Sheet was 7K cooler than the actual data indicates. It's not clear whether this is an issue with regional details, or an indication that the best global fit isn't actually a very good fit.

The authors offer three explanations for why their results provide a lower climate sensitivity than previous work: their temperature reconstruction suggests the LGM was warmer than earlier work; their temperature data comes from sources that are a bit more evenly distributed around the planet; and not all studies have included estimates of atmospheric dust.

This looks like a solid effort, and should be easy to extend to using other climate model ensembles, which would provide a greater degree of confidence in their climate sensitivity value (assuming that other models agree, of course). But, as the authors note, their results are sensitive to the estimates of global temperature, which in turn are dependent on where we've obtained information about the LGM. More widely dispersed sources of data from this time period would be the clearest way of improving all the estimates derived from this time.

The other thing worth noting is that there may be limits to how much a climate sensitivity from the past can tell us how the Earth will behave now. Because of different feedbacks and starting conditions, it's possible that the climate sensitivity at the height of a glacial era will be slightly different from that of an interglacial era like the one we're now in. Fortunately, whatever conditions that created a snowball Earth millions of years ago don't seem to currently apply.